The present invention relates to a magnetic recording medium, more particularly to magnetic tape having a back-coat with improved running property and durability.
Magnetic recording media such as magnetic tape, sheets and disks are extensively used in audio, video and computer applications. Magnetic tape is often used in cassettes. Video recording and playback are performed with a cassette which is transported on a video deck and scanned in rubbing contact with a magnetic head as it is guided by guide poles and rollers. In order to improve the tape sensitivity, especially the output of video recording and playback in the high-frequency range, the tape must have a stable rubbing contact with the magnetic head, and to meet this requirement, the magnetic layer is finished to have a smooth surface. However, when the tape is transported on the video deck, both the obverse and reverse surfaces of the tape are abraded by the guide poles and rollers, and the good running property of the magnetic layer on the obverse surface does not ensure the absence of an excessive tension on the tape if the rubbing area of the reverse surface does not have good runnability. If an excessive tension is applied to the running tape, the magnetic layer is rubbed with the head so vigorously that the layer is damaged or magnetic particles shed from the layer. As a further disadvantage, varying pressures by which the tape is wound on a takeup reel cause uneven tape running during repeated operation. These defects are all harmful to video or electro-to-magnetic conversion characteristics and cause skew, jitter or reduced S/N.
In order to avoid these problems, a backcost is usually provided on the reverse surface of magnetic tape. While various methods are used to improve the ability of the back-coat to run smoothly in contact with guide poles and rollers, a typical technique is by incorporating inorganic particles in a resin layer. The essence of this technique lies in roughening the surface of the backcoat so that it has a reduced area of contact, hence a smaller coefficient of friction, with guide poles and rollers. The use of inorganic particles in a resin layer are taught in many prior art patents such as Japanese Patent Application (OPI) Nos. 13034/1982, 161135/1983, 53825/1982 and 2415/1983 (the symbol OPI as used herein means an unexamined published Japanese patent application). Most of these patents specify the size of the inorganic particles used; for example, Japanese Patent Application (OPI) No. 130234/1982 shows using inorganic particles having an average size of 0.02-0.5 .mu.m to provide a backcoat having such a surface roughness that the center-line-average roughness, Ra, is not more than 0.024 .mu.m for a cut-off value of 0.08 .mu. m. Japanese Patent Application (OPI) No. 161135/1983 shows the use of similar inorganic particles except that Ra of the resulting overcoat is 0.025-0.1 .mu.m.
However, even the use of inorganic particles in the prior art is not able to provide sufficient running property and prevent the occurrence of excessive tension on the running tape. Furthermore, the surface of the backcoat is damaged as a result of cyclic use of the magnetic tape. These defects are presumably due to the high frictional coefficient of the surface of the backcoat, which not only increases the frictional resistance of the tape but also causes the resultant increase in the force of rubbing the backcoat, thereby making it sensitive to undesired surface abrasion.
If the backcoat does not have good running property or durability, the magnetic tape is unable to run smoothly on a tape deck and the electro-to-magnetic conversion characteristics mentioned above the magnetic layer may be impaired. With the recent introduction of portable, compact and high-density recording VHS or beta-video movies, the practice of video tape recording has expanded to outdoor locations, and the development of magnetic tape adapted to such applications is desired. To this end, more rigorous requirements must be met. First, compact and high-density video equipment has a complex tape transport path and the magnetic tape is brought into rubbing contact with guide poles and rollers at many points, and therefore, the tape is required to have a high resistance to abrasion. Secondly, a hot and humid atmosphere may occur in the outdoor application, so the tape is also required to have high resistance to such hostile conditions. In short, magnetic tape adapted to use with compact and high-density video recording equipment must have more-than-usual running property, resistance to shedding, and durability.
The conventional backcoat uses inorganic particles having an average size in the range of 0.02-0.5 .mu.m and it has various problems to be solved before it can be applied to the magnetic tape adapted to compact and high-density video recording equipment.